Automatic parking brakes (APB) typically include an operating element such as, e.g., a push-button switch by which the parking brake may be locked or released. When the operating element is actuated, a control unit connected to it recognizes the parking brake command and accordingly controls an actuator such as, e.g., a hydraulic pump or an electric motor in order to build up braking force on the wheels of the vehicle or to release the brake.
The following considers systems in which electric motors (having gear units) are situated directly on the wheel brakes (so-called “motor on caliper”). Such parking brake systems are typically designed so that the driver may operate them when the vehicle is at a standstill and while driving. The possibility of an operation while driving is provided particularly for the purpose of being able to brake the vehicle in the case of a defective service brake. In this case, the vehicle is braked with a predefined deceleration, e.g., 3 m/s2. For this purpose, the deceleration of the vehicle is usually controlled.
A difficulty of such parking brake systems using electric motors is that the response of the mechanical components is substantially slower than in a hydraulic parking brake system. This results in a relatively poor controllability of the electromechanical systems. This may cause the braked wheels to lock too long while being controlled. In a critical driving situation, for example, when oversteering or understeering while cornering, a vehicle may lose its cornering force.